1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a magnetic recording device, such as a magnetic tape device or magnetic disc device, and also to a method of producing such a magnetic recording medium.
2. Description of the Prior Art
Recently, magnetic disc devices have been extensively used as external memory devices for information processing devices such as computers. FIG. 1 shows a schematic cross-sectional view of a conventional magnetic recording medium for use in such a magnetic disc device, disclosed in Japanese Patent Application Laying-Open Nos. 63-79233 and 6379234. The magnetic recording medium shown in FIG. 1 comprises a substrate 1 composed of a non-magnetic substrate plate 11 and a non-magnetic metal layer 12 formed thereon, and further a primary layer 2 of non-magnetic metal, a thin-film magnetic layer 3 of a ferromagnetic alloy, and a protective layer 4 of amorphous carbon are laminated on the substrate 1. Further, a lubricant layer 5 made of a liquid lubricant is formed on the protective layer 4.
This conventional recording medium is produced according to the following procedure. Namely, the non-magnetic metal layer 12 of a Ni--P alloy is formed by electroless plating on the non-magnetic substrate plate 11 made, for example, of an Al alloy and finished to predetermined parallelism, flatness and surface roughness. Then, the surface of the layer 12 is polished so that the non-magnetic substrate 1 has a predetermined surface roughness. The non-magnetic substrate 1 is heated to 200.degree. C., and the non-magnetic metal primary layer 2 of Cr having a thickness of 3000 .ANG., the magnetic layer 3 of Co--Ni--Cr alloy (Ni:30%; Cr:7.5%) having a thickness of 500 .ANG., and the protective layer 4 of amorphous carbon having a thickness of 200 .ANG. are sequentially formed on the surface of the substrate 1 in a laminated manner by a sputtering process. Further, a fluorocarbon-type liquid lubricant is coated on the protective layer 4 to form the lubricant layer 5 having a thickness of 20 .ANG.. The recording medium thus formed has mechanical characteristics such as strength and dimensional accuracies good enough not to invite any trouble in practical use. The magnetic characteristics of this medium are also good, and more specifically the coercive force Hc is about 1000 Oe, and the squareness ratio Br/Bs is about 0.80 to about 0.85.
U.S. Pat. No. 4,789,598 discloses a vertical recording-type magnetic recording disc in which a primary layer of chromium, a vertical magnetic anisotropic layer of Co--Pt--Cr alloy containing not less than 17% of Cr, and a protective layer are sequentially formed on an aluminum substrate having a surface layer of NiP.
However, recently, the requirements for mass-processing of information have increased and diversified, and it has strongly been desired that magnetic disc devices should have high recording densities and large capacities. Therefore, it has been necessary to provide a magnetic recording medium which has a higher coercive force Hc and a high squareness ratio Br/Bs commensurate with a thin-film magnetic head used for achieving a high recording density. In magnetic tape devices, it has also been desired to achieve such a high-recording density design.